There is well known an image display apparatus of a head-mounting type (head-mount display) that uses an image display device such as a CRT or an LCD to enlarge to display an image displayed on the display device via an optical system.
An image display apparatus such as the head-mount display is, in particular, desired to be miniaturized and lightened as the apparatus as a whole in order to be mounted on the head. In addition, when a weight balance, an appearance and the like are taken into account, it is desirable to make the apparatus thin in a visual axis direction of an observer. Moreover, an image is desirably enlarged as much as possible in order to make an enlarged image to be displayed powerful.
FIG. 26 shows a conventional image display apparatus using a coaxial concave mirror. In this apparatus, a light beam from an image displayed on a display device 101 is reflected on a half mirror 102 and caused to be incident on a concave mirror 103, and the light beam reflected on the concave mirror 103 is guided to an eye E of an observer via the half mirror 102. The image displayed on the display device 101 is formed as a virtual image enlarged by the concave mirror 103. Consequently, the observer can observe the enlarged virtual image of the image displayed on the display device 101.
In addition, for example, in Japanese Patent Application Laid-open No. 7-333551 (European counterpart Patent No. EP687932A2), Japanese Patent Application Laid-open No. 8-50256 (European counterpart Patent No. EP687932A2), Japanese Patent Application Laid-open No. 8-160340, Japanese Patent Application Laid-open No. 8-179238 (European counterpart Patent No. EP687932A2) and the like, there is proposed an image display apparatus that uses an LCD (liquid crystal display) as an image display device for displaying an image and a thin prism as an observation optical system to realize thinning of the entire apparatus.
FIG. 27 shows an image display apparatus that is proposed in Japanese Patent Application Laid-open No. 7-333551. In this apparatus, light emitted from an LCD 111 is incident on an incident surface 113 of a small eccentric prism 112. Then, a light beam is returned between a total reflection surface 114 and a reflection surface 115 which are formed in the prism 112 and have curvatures, and thereafter, exits from the eccentric prism 112 through the surface 114 and guided to an eye E of an observer. Consequently, a virtual image of an image displayed on the display device (LCD) 111 is formed, and the observer observes this virtual image.
The reflection surface 115 of the eccentric prism 112 is constituted by an eccentric rotation asymmetrical surface (a surface having different optical power depending on an azimuth angle, a so-called free-form surface).
A type of an optical system shown in FIG. 27 has a characteristic that it is easy to make the entire apparatus thin and make an angle of view of an observation field of vision wide compared with a type using the conventional coaxial concave mirror shown in FIG. 26.
In recent years, definition of an LCD or the like that is a display device for displaying an image has been improved, and an LCD or the like has been developed which is miniaturized while having the same number of pixels compared with that in the past. When such a miniaturized image display device is used, although it is advantageous for miniaturization of an apparatus, it becomes necessary to increase a magnification of an optical system in order to attain the same angle of view as in the past.
In view of such circumstances, Japanese Patent Application Laid-open No. 10-153748 proposes an optical system in which an eccentric prism and a relay lens system are combined, and an intermediate image is once formed by the relay lens system, and then, an image displayed on a display device is guided to an observer. Consequently, a magnification is further improved and an angle of view is widened with respect to an LCD size while maintaining the characteristic that the entire apparatus is thin as shown in FIG. 27.
In addition, as an optical system with more improved optical performance compared with the optical system proposed in Japanese Patent Application Laid-open No. 10-153748, an optical system of a type in which number of internal reflection surfaces of an eccentric prism is increased to form an intermediate image only by the eccentric prism and the image is guided to an observer, an optical system of a type in which a second eccentric prism is added to a first eccentric prism optical system, and the like are proposed in Japanese Patent Application Laid-open No. 2000-066106 (U.S. counterpart patent No. 6,310,736BA), Japanese Patent Application Laid-open No. 2000-105338, Japanese Patent Application Laid-open No. 2000-131614, Japanese Patent Application Laid-open No. 2000-199853 (U.S. counterpart patent No. 6,201,646BA), Japanese Patent Application Laid-open No. 2000-227554 and Japanese Patent Application Laid-open No. 2000-231060.
In general, an optical system of a type in which an intermediate image is once formed has a problem in that an optical path length is increased and an apparatus becomes large. However, in the optical system proposed in each of the above-mentioned patent applications or patents, miniaturization is attempted through contrivances such as using a surface for carrying out both of a transmitting action and a reflecting action or making light paths intersect.